This laboratory has recently discovered a new enzyme and purified it to homogeneity from mouse lung. Studies of substrate specificity and mechanism have shown this enzyme to be S- adenosylmethionine:thioether S-methyltransferase (TEMTase), although it also methylates selenium and tellurium atoms in seleno- or telluroethers. The long-term objectives of the proposed research are to characterize the molecular properties of TEMTase and to define its physiological role in sulfur and selenium metabolism. Potential thioether and selenoether substrates for TEMTase are known to be ingested as environmental contaminants or to be generated metabolically from xenobiotics or excess nutrients. The specific aims and experimental design are: 1) to further study the substrate specificity of TEMTase including thioethers known to be ingested or to be generated in vivo; 2) to derive the amino acid sequence of TEMTase by base sequence analysis of cloned cDNA: 3) to prove the active site of the enzyme by affinity labelling with the photoactivatable 8-azido- analog of S-adenosylmethionine, and with two vinyl sulfonium compounds, methyl ethyl vinyl sulfonium and S-adenosylvinthionine; 4) to define the enzymology of selenium speciation by a combination of in vivo metabolic studies and in vitro reconstruction of selenium methylating systems from purified components; 5) to study the role of TEMTase in metabolism of glutathione conjugates and the related methylthio derivatives of xenobiotics; 6) to study the role of TEMTase in metabolism of methane thiol produced from D-methionine or excess dietary L-methionine; and 7) to study the species distribution of TEMTase in selected animals, plants, and microbes using enzymological and immunochemical measurements. The results of this research will aid in understanding the metabolism of the essential trace element, selenium, which is also toxic at higher levels. Additionally this research holds the potential for establishing a central role for TEMTase in a novel pathway for the metabolic generation of xenobiotic thioethers and/or their further methylation to methyl sulfonium derivatives.